This invention relates to a duty ratio control method for controlling a duty ratio with which a solenoid valve means is to be driven, and more particularly to a method of this kind which is adapted to perform control of a solenoid valve means so as to control the flow rate of a fluid to be regulated by the solenoid valve means to a desired value in a prompt and accurate manner.
A control method is widely known which controls the flow rate of a fluid flowing in a fluid passage by means of on-off control of a solenoid valve arranged in the fluid passage.
In such control method, in order to control the flow rate of fluid to a desired or target value promptly upon setting of the same value as a new value, without overshooting or hunting of the flow rate of fluid, it is necessary to set the duty ratio with which the solenoid valve is to be operated, in accordance with the desired flow rate value and the difference between the actual flow rate value and the desired flow rate value. The duty ratio setting requires previously determining with accuracy the flow rate characteristic of the fluid flowing through the fluid passage relative to the duty ratio of the solenoid valve. However, this requires a great deal of time and labor if the solenoid valve is produced on a mass production basis. That is, there can occur variations in the flow rate characteristic of the fluid between the individual solenoid valves, attributable to machining tolerances of component parts including the solenoid valve and the fluid passage, fitting and assembling tolerances thereof, aging changes in the performance thereof, etc., making it difficult to determine with accuracy the flow rate characteristics of individual solenoid valves and also to perform such determination periodically or each time a predetermined period of use lapses. If to avoid these difficulties, the duty ratio for the solenoid valve is set in accordance with an average flow rate characteristic of the solenoid valve, it is impossible or takes much time to control the flow rate of fluid to a desired or target value with accuracy in the case of a solenoid valve having a different flow rate characteristic from the average one, even causing overshooting or hunting of the flow rate of fluid.